Mass spectrometer



Nov. 17, 1953 LEE 2,659,822

MASS SPECTROMETER Filed April 22. 1947 To Measuring Circa/z /7'" INVENTOR.

George H Lee I Patented Nov. 17, 1953 UNITED STATES MASS SPECTROMETER George H. Lee, Oak Ridge, Tenn., assignor to the United States of America as represented by the United States Atomic Energy Commission Application April 22, 1947, Serial N 0. 743,115

9 Claims. (Cl. 250-413) My invention relates to isotope separating equipment, and more particularly to massspece trometers employing alternating electrostatic fields forthe separation of isotopes of elements.

In the prior art of isotope separation it has generally been the practice to heat a charge of material until it vaporizes, lead the vapors to an ionizing chamber, subject the neutral vapors to electron bombardment therein to ionize them, eject the ions from the ionizing chamber with accelerating potentials to bring them into the region of a magnetic field where they are caused to travel in arcuate paths of radii corresponding to their masses, and finally to collect the respective ions ator near the focal points of the ion beams. The structure and framework for setting up the magnetic field is generally bulky and cumbersome, and careful precautions must be taken to provide a uniform magnetic field to insure proper operation- The measurement, maintenance, and reproduction of a constant and homogeneous magnetic field is, therefore, often difiicult and impractical.

Some efforts have been made in the prior art to provide a mass spectrometer employing an electrostatic field, set up by "a high frequency alternating current, at right angles to the ion beam, instead of using a magnetic field. Without changing the frequency of the alternating current source and by utilizing harmonics of the same .frequency, it is possible to improve accuracy in the separation, collection, or comparison of certain isotopes without introducing cumulative errors, since frequencies of the harmonics of an electric circuit are exact multiples of the fundamental to a high order of precision, and difierent harmonics may b employed to deflect ions of different mass. Examples of these efforts are set forth in Physical Review, vol. 28, pages 1275 to 1286, and Physical Review, vol. 40, pages 429 to 433. These systems, however, do not contemplate any arrangement for increasing the angular deflection of the displaced. ions and in the case of heavy ions would require considerable spacing between the electrostatic field and the receivers to obtain the necessary displacement Also, under these latter arrangements relying on alternating current deflecting fields, the angularly displaced ion beams will sweep back and forth across the receiver for the desiredisotope and cause contamination, and the sweep of these ion beams back and forth across such receiver makes separation and collection of the sweeping beams into separate receiversimpractical... a

Applicant with a knowledge, of these defects; in and objections to the prior art has for an object of his invention the provision of isotope separating equipment with an alternating elec-f reduce the lengths ofthe paths of travel of the ions from the field to the collector.

Applicant has as another object of his inven-l tion the provision of an alternating electrostatic field .mass spectrometer employing out of phasealternatingcurrents to produce a rotating field so that a plurality of different isotopes may each be collected in spaced concentric circular res. ceivers, or receivers forming sectors of circles of radii corresponding to their respective masses; Applicant has as another object of his inven-i' tion the provision of an alternating electrostatic field mass spectrometer employing alternating current potentials whereby selected isotopes of elements may be chosen for deflection and measurement and undesirable isotopes may pass there through substantially undefiected.

Applicant has as a further object of his invention the provision of an alternating electrostatic field mass spectrometer which produces alternating current pulses on the collectors when spaced sectors set in a plurality of circular tracks with radii corresponding to the respective masses of the isotopes to be measured, are employed. Applicant has as a further object of his in-'- vention the provision of a mass spectrometer which produces alternating currents at the 001-. lcctors and provides through amplification and conversion -more accurate measurement of th isotopes collected. 7

Applicant has as a still further object of his in-. vention the provision of a mass spectrometer which is particularly well adapted to the separation and analysis of isotopes of elements of the lower mass numbers. I Applicant has as a still further object of his in--' ventionthe provision of an alternating electrostatic field mass spectrometer for selecting a de-. sired isotope of a predetermined mass and causing it to travel undeflected to a collector or receiver. Other objects and. advantages of my inven-l tion will appear from the following specification and the accompanying drawings, and the novel features thereof will be particularly point-.. ed out in the annexed claims. 'I In the drawings, Fig. 1 is a schematic of one arrangement of my improved mass spectrometer.

Fig. 2 is a schematic of the ion beam deflection arrangement for a modified form of my improved mass spectrometer. Fig. 3 is a schematic of a portion of my improved spectrometer employing the arrangement of Fig. 2. Fig. 4 5 is a schematic of the ion sectors and the circuit for making alternating current measurements of isotope currents: Fig. 5 is. a schematic of another modification ofthe arrangementfor de fleeting the ion beams.

Referring to the drawings in detail, desig'-" nates a positive ion source of any conventional: type such as is well known tothe mass spectrometer art, wherein a suitablbharg'eis-vaporized by heating, and the vapors therefrdin" are" fed to an ionizing chamber providedwith" anexit' slit. In this chamber they are subjected to clear tron bombardment and are ionized by a stream of electrons emitted from, a cathode and dir'ected across the chamber. The ion source I is pref- 2n i'ably maintained athigh potential, such-as 2 7 'k'v., from a suitable direct-current source, cated 'at' Positioned beyond. the sourcei'l' s'pacedsets of 1 accelerating electrodes: 2, 2 having aligned slots which' are in turn aligned. withthe'exit -slit in the" ion source; These ac? celeraitingelectrodes are preferably joined to gthei" electrically a'nd grounded in order to maintain the 1011' team t a high constant hori- Zciitaf-velocity" during its -p'assage between plates 3*: I] virliich are interposed betWe'enthe electrodes 22 fa ndma-inta'in a vertical auernatmg memo static field therebetween. Plates 3 3 are fedflfrom 151i ""ends er the centrally tapped; secondary or transformer" rwith an alterna'tingcurrentyolt a aying a maximum' value-ofl-forexample;

vi, delivered'by-any suitable alternating Scur- "source. Thecentertap of the'secondary winding" is grounded The frequencyofabout c fziiegacydes'a pned acros's'p1ate's 3',- 3 is chosen 40 so tli'afthe transit time through the plates 3 -3 fo he' desiredions, suchasuraniuin" 235; is eiiactly-edua'l tb the periedora multiplethereof, preferably one complete cycle of the alternating ciirreirit applied te such plates.

Positioned beyond the accelerating electrodes 22 fis'adeceleratirig electrode 5' h ving a slot die; lig'hed with theslots off'th'e acceleratin e1 trod 1 "The 'decelerat'irig' metmae's may e mama ed are; high potential 6f -about-27 ,2- 25' 5O riemseurce-s and this pot'ential-isonly 1y iessuian' that of the "p'o'sitive its source Positionedon beyond the deceleralting electr e 5 and "separated therefrom by space 6 'i s a eifTh'avingafcollectororreceiverB which is in" substantial axial alinement with the slots; the electrodes- The screen and collector are-elecally" connected" to" and' maintained at substantiallythe same potential as the decelerating electrodet; Amnieter l5 servestomeasurethe 1 ciirr'ent resulting from the totalioni collection-on the screen and collector an'd ne'utraliaation'thereof. orcours'e; the'c'olle'ctor couldbe insulated ibm the screen and have; a: separatemetered r'nfleadtothe'supply,jif desired; whereby the 35 ion" our'rentco'uld be'separately" measured; "In" its operation a charge containing the isotopes to be separated is vaporized and the-vapors idnized in sourcejl. The acceleratingelectrodes 2; 2 eject the ions from the exit slit in-the source I and movethem towards thescreen Tthrough pistes 3 31 Where the frequency of'ithea'lternating current potential appliedto plates 3,; 3 so chosen that 1 the transit time therethrough a multiple thereof, the integral of the vertical acceleration of the 235 ions during their passage through such plates is, therefore, equal to zero. Accordingly, all uranium 235 ions emerge with a zero vertical velocity. The uranium 238 ions, having a slower horizontal velocity than the uranium 235 ions, wil l require slightly more than a complete cycle to pass through plates 3, 3 and willemerge therefrom with a small vertical velocity. This vertical velocity Will vary sinusoidally for successive 238 ions depending upon the time oft their entry into the electrostatic field between plates 3, 3. 7 Hence, the 238 ion beam will squar -vertic lly about the 235 ion beam.

The horizontalgvelocity of the ion beams may be retiu'cedby 'soniefactor, say 10, upon reaching the "decelerating electrode 5, by adjusting its potential to the desired value, in order to accentuatethe effect of the vertical velocity of the 238 ions. The ions are allowed to drift in space fi until th'e' desiredseparatiori is had. By, theme of such decelerating:electrodezthe space requiredfor vertical separation is greatly. reduced and-the: size-of the equipment'is brought within reason-= able limits.

Underthe above. conditions,- using. the indi cated voltages and-frequencies, lengths: of plates: 3, 3 equalzto2.5oentimeters, with 1 centimeter separation-' between; plates, and a driftdistance: 6 'o'f-90 centimeters; themaximumvertical dis-- pl'acement of the 238-Lion beam'from the 235 -ion beam in one":direction-' atithe collector is. about: 6 centimeters; If desired, the 235ion beam could beaccelerated again andpassed through asecr 0nd stage. It will beiundersto'odzthata the r-useof: a-single pair' of plates of:sufi1cient'length to cover? awhole-cycle' is only" one. embodiment of my in-- vention; and that a'pa'irrof spaced" plates which. will 'only cover symmetrical or corresponding .but opposite portions ?of the cyc'le would also be. effec'e tivefor thefpurposea l'r'i the embodiment of Fig; 1 completesepae ration of the isotopes cannotibeiobtainedzbecause. of thei necessity for: the 238 ion beam to sweep across the 235 ion 'co'l'1ector '8. Although the rs.- sulting; contamination would likely be extremelysmall,- it could be entirely eliminatediby. the.alter.-- nating current plateiarrangement:shown inEigs'.v aan'dc:

lnlthat modification', transformers I'l I24 are: or. the? center tapped secondar type similar: to the: one employed in Fig; 1. These: transform.- ers a'refed by" alternatingzcurrent.sourceszofaany suitable type: 501 that they deliver: voltages. to 1ate's |s I3 and=l4 I4 which-are; outtof: phasez with each: other. It will be. seen fromi liigsz- 2 and 3 tha't-p'lates l 4'1, M liewin-ispacedverticalplanes' whilespla'tes I 3 l3 lie.in spaced-horizon;- ta-l planess Under these conditionsthe-238; ion beam; instead :of: oscillating will now rotate about a circletwhicn Will have/a radius" ofi-aboute eentimeter s at thercollector'since arotating electrosstatic:fi'eldzwill ber set up by 1 the alternating cur rent potentials impressediupon*plates l3 i 3' and l4-':,. I41. A1:circularcollectoror'receiver 96' may beremployed to collect the:- uranium 238 isotope; In this arrangement: theluranium 235; isotope may bezcollected'in rec'eiver'8'.

From: the foregoing it'- will become apparent that-instead otemploying the central collector 8-" for the desired orchosen isotope," it maybe desirable: particularly withv elementsof low mass numbers; to pass: the undesired} or'uncho'sen; isot'opes -through axia1ly without appreciable-dcfiection, and deflect the desirecl or -chosemiso topes', collecting them in circular collectors, or

measuring them as a direct current by simply sufiicient deflection can be obtained without recourse to this deceleration, electrode could and should be omitted without in any way adversely affecting the operation. Of course, if the decelcrating electrode 5 were omitted, screen I, and the'various collectors should be grounded.

In the modification of Fig. 4 sectors [5",15" and l6", [6" are employed to replace the collectors, such as [6 of Fig. 3. As the ion beams rotate in conformity withrotating electrostatic fields setup by the plates, they pass from one of the pairs of sectors l5", [5" or [6", Hi to the other. The currents thus produced are led through leads 22", 22" to the resonant circuit including capacitance l9 and inductance in the form of the primary of transformer I8", and through leads 23", 23" to the resonant circuit including capacitance 2 I and inductance in the form of the primary of transformer 20'. The secondaries of these transformers feed alternating current measuring equipment. Thus with the split circular rings or collectors, the ion currents may be measured as an alternating current by feeding the pulses into resonant tank circuits. This is advantageous in that alternating current can be more easily amplified and measured than direct current.

In the foregoing arrangements, although the 235 ions will emerge from the electrostatic field with zero vertical velocity, they will be vertically displaced slightly, the vertical displacement varying sinusoidally with time. For instance, in Fig. 1, the 235 ion beam will actually oscillate slightly about the straight line path shown. This displacement modulation will also be superimposed on the velocity modulation of the 238 ions. Its maximum computed value in both cases is about .5 centimeter for the previously given dimensions and voltages. This displacement modulation could be cancelled out if desired by employing a second set of plates l'l, l'l', axially displaced horizontally from the first set of plates 3", 3" as shown in Fig. 5. Each 235 ion will enter plates Il, I'l exactly one-half cycle later than it enters plates 3", 3" and will receive an equal and opposite displacement. The 235 ion beam will therefore emerge from plates I'I', 11'' with zero vertical velocity and zero vertical displacement. Each 238 ion, because of its slower horizontal velocity, will enter plates IT', IT slightly more than one-half cycle later than it entered plates 3", 3"; therefore, the vertical velocity and displacement modulation it receives in plates 3", 3" will not quite be equal to that received in plates ll, l'l'. Accordingly, the 238 ions will still emerge with a vertical velocity and displacement although both will be of a smaller magnitude than results from the arrangement of Fig. 1.

While the foregoing invention has been largely illustrated in connection with the separation of uranium isotopes, it will be understood that it may be employed for general isotope separation, whether the materials utilized are fissionable or not. ,For'example, the isotopes 63 and 65 of 001)- per or the isotopes of any other element maybe equally well separated by the foregoing arrangements. i 1

Having thus described my invention, I claim:

1. An isotope separation system of the character described comprising an ion source for producing ions from neutral vapors, means for accelerating the ions into a transverse alternating electrostatic field for selectively deflecting ions of different masses, collectors for said ions spaced from said field, and means for decelerating the ions leaving said field for accentuating their deflection while travelling through the space to the collectors.

2. An isotope separation system of the character described comprising an ion source for producing ions from neutral vapors, a source of alternating potential and a pair of spaced-electrodes for establishing a transverse electrostatic field for deflectin ions passing therethrough, accelerating electrodes positioned on either side of the field for ejecting the ions from the source and moving'them through the field at a uniform velocity, a decelerating electrode for accentuating the deflection of the ions, and means spaced from the field for collecting and measuring said ions.

3. An isotope separation system of the character described comprising an ion source for producing ions from neutral vapors, means for establishing a transverse alternating electrostatic field spaced from the ion source, accelerating electrodes positioned on either side of said field for ejecting the ions from the source and moving them into the field at substantially right angles thereto, said field being synchronized with the movement of the ions of a predetermined isotope for deflecting the ions of other isotopes passing therethrough into separate paths, a decelerating electrode for accentuating the deflection of the ions, and means spaced from the field and in the paths of said ions for measuring the ions so separated.

4. An isotope separation system of the character described comprising an ion source for producing ions from neutral vapors, a transverse rotating electrostatic field, means for applying an accelerating potential to said ions for causing them to pass therethrough, said field deflecting the ions of respective isotopes into respective paths which periodically describe circles, and circular collectors positioned in the paths of travel of said ions for collecting them.

5. An isotope separation system of the character described comprising an ion source for producing ions from neutral vapors, a transverse rotating electrostatic field, means for subjecting the ions to an accelerating potential for moving them into said field, said field being synchronized with the movement of the ions of a predetermined isotope for deflecting the ions of other isotopes into paths which periodically describe circles, and circular shaped collecting means positioned in the paths of travel of the ions for collecting them and measuring the resulting current therefrom.

6. An isotope separation system of the character described comprising an ion source for producing ions from neutral vapors, a transverse rotating electrostatic field, means for subjecting the ions to an accelerating potential for moving them into said field, said field being synchronized with the movement of the ions of a predetermined isotope for deflecting the ions of other isotopes into paths which periodically describe circles, means beyond the field for subjecting the nsto theacticnci; a oecelerotins rq fi 19% accentu te the r. deflect on, nd..,. ing said qns 7. An isotope separation system of the 911,341;- acter de cri d comprising a i n; s urc 1 fo produci ions ro neutra v ors-e t ve ansicro llecte rotating electrostatic field ;-.spaced from the, ion.

source, accelerating electrodes positioned I on ith r side of. the fieldv for ieeting ions irom the source and oan ingthenr tcmove through theiield, said field being;synchronized-with the.

movement of the ionsgf a predetermined isotope for deflecting the QnS of-other isotopesi lto, paths which periodically describecircles; means for an.

plying a decelerating potential to the ionstafter they leave the field to accentuate thedeflection,

and concentric circular collectors spaced from the.

circles-spaced iron; :the fieldior. produc n .9. pnlsntinsi ncurre t. cor sponding to thenuina ber ,oiions i-.:the. respective isotop s strikingf them. 9, 'An isotope separation system of the char.-

actendescribedcomprising an ion source for pro-.

ducing ions .frolnneutral vapors, spaced setsnof opposed plates for settingup a t natin ic fi lds, ans-foraccelerating. said ions.be-. tween said plates, said plates being so positioned and the fieldsthereof beingso synchronized with the movement Of-theions of. apredetermined' isotope that such ions leave the field .withoutapw prec able d flecti W ile ions of, other. isotop s are deflected intoseparate paths, means for de-, celerating. said ions after they pass through said plates andmeans, spaced from they field for 0011-. looting the ions leaving the field.

GEORGE .H. LEE.

References .Cited. inthe file of this patent.

Smythe: PhysicaljReview, v01. 28,-December, 1926', pp. 1275-4286.

Smythe and Mattuch; Physic l Review, vol. 40, 

